Broken bones heal naturally, albeit slowly compared to most soft tissue, provided they are adequately supported and relieved of stress. In a simple break in an extremity, adequate support and relief may be provided from outside the body with a device as simple as a splint or a cast, which immobilizes the body part containing the broken bone. Such procedures may suffice when the bone can be set and will retain its position without significant intervention, for instance when the break is simple and contained in a body part that can be readily immobilized in a natural posture. Immobilization is also therapeutic to treat damage to connective tissue, by preventing repetitive stress and further injury to, for instance, damaged ligaments, tendons, or cartilage.
When a break or fracture is in the spine, or when the connective tissue between one or more vertebrae is damaged, external immobilization is significantly less effective for several reasons. Because the spine is the central support column of the human body, externally imposed immobilization is impractical, as it involves immobilizing most of the body. Furthermore, the spine is a load-bearing structure that is subject to repetitive compressive and rotational stresses constantly during the normal waking life of a person; therefore, external immobilization of the spine significantly impacts the mobility and activity of a patient. For practical purposes, externally imposed spinal immobilization often requires that the patient is subjected to bed rest, is wheelchair-bound, is fitted with a significant amount of uncomfortable stabilizing equipment, or a combination of the above.
Since the advent of sterile surgery, it has been possible for doctors to internally stabilize broken bones and connective tissue with implants. Internal stabilization can be complex, but tends to allow much greater precision in aligning broken bones, and significantly reduces misalignment in healing. Internal stabilization also improves healing time and allows a patient to live a much more normal life while still healing. One such type of implant is a bone plate, which is a shaped rigid or semirigid part usually having several through-holes by which a surgeon will attach the plate to parts of a broken bone, or to parts of two or more proximate bones that require alignment, by means of screws. All such parts are formed of biocompatible materials and may either be left in the body during and after healing, or may be removed after healing. Ideally, bone plates would be painstakingly formed and attached in several directions, so that the plate would conform perfectly to the patient's body, and would be secured to the bone or bones with an optimal balance of minimal tissue damage and maximal rigidity. In practice, the fact that such devices must be attached in surgery restricts the amount of time and the amount of access to the bone, such that physicians require such devices to attach efficiently and primarily from one direction.
Another significant challenge to the use of bone plates is the stress placed on the bone by the tightening of the bone screws. Ordinary screws in other fields may be held fast to a surface by the friction between the screw head and the outer surface of the attached part, by friction between the screw threads and the material, or a combination. However, the force generated by tightening screws to achieve such friction in bone may cause excessive damage, and the healing of the bone over time in combination with the motion of the body may act to gradually force the bone screw from its position. Therefore, bone plate implants may require an assortment of apparently contradictory features including but not limited to additional locks to prevent the extrusion of the bone screw from the bone and plate, attachment that is both secure and that provides some wiggle-room, attachment that is very quick but also very secure or conforming, and/or other features.
Anti-backout mechanisms on bone plates tend to suffer a variety of drawbacks. Parts of conventional anti-backout mechanisms, for instance screws and washers, tend to be small and delicate, and can be broken during installation or lost by the surgeon within the surgical wound. In conventional bone plates that possess internal anti-backout mechanisms, securing the mechanism may require specialized tools, or it may be difficult to ascertain whether the anti-backout mechanism has been fully engaged.